SAN FRANCISCO/SEOUL (Reuters)- Apple Inc has sued rival Samsung Electronics claiming that the South Korean firm's Galaxy line of mobile phones and tablets "slavishly" copies the iPhone and iPad, according to court papers.

Samsung is one of the fastest growing smartphone makers and has also emerged as Apple's strongest competitor in the booming tablet market with models in three sizes, although it remains a distant second in the space.

Its Galaxy products use Google Inc's Android operating system, which directly competes with Apple's mobile software, the lawsuit said. However, Apple's claims against Samsung focus on Galaxy's design features, such as the look of its screen icons.

John Jackson, an analyst with CCS Insight, said Samsung is essentially Apple's only real tablet competitor at this stage.

"It's clear that they do not intend to let Apple run away with the category," Jackson said.

"This kind of blatant copying is wrong," Apple spokeswoman Kristin Huguet said in a statement.

Samsung said in a statement on Tuesday that it would respond to the legal action "through appropriate legal measures to protect our intellectual property."

"Samsung's development of core technologies and strengthening our intellectual property portfolio are keys to our continued success," it said.

To better compete with Apple, Samsung redesigned within weeks its new 10.1-inch tablet, first introduced in February, to make it the thinnest in the category after Apple set the trend with its iPad 2.

Apple CEO Steve Jobs has criticised Samsung and other rivals in presentations of new products or technology debates. Analysts say the response by Samsung to this has been muted, partly because Apple was Samsung's second-biggest customer last year after Japan's Sony.

Apple brought in around 6.2 trillion won ($5.7 billion) of sales to Samsung in 2010 mainly by purchasing semiconductors, according to Samsung's annual report.

Samsung shares slipped about 1 percent in early Tuesday trade to their lowest level in one month, underperforming a 0.5 percent fall in the broader market. At 8:40 p.m. EDT, Samsung shares were down 0.7 percent at 861,000 won.

WEB OF LITIGATION

Apple is one participant in a web of litigation among phone makers and software firms over who owns the patents used in smartphones. For instance, Nokia has sued Apple, which in turn has sued handset maker HTC Corp.

U.S. International Trade Commission staff, at a hearing on Monday, recommended that HTC and Nokia shouldn't be found liable for infringing Apple's patents relating to smartphones, Bloomberg reported.

ITC staff act on behalf of the public and its recommendations are not binding.

In its lawsuit against Samsung, Apple noted that earlier versions of Samsung smart phones did not embody the same combination of Apple's designs.

"Even the icons in earlier versions of the Samsung smart phones looked different because they had a variety of shapes -- and did not appear as a field of square icons with rounded corners," the lawsuit said.